Instruction manual

AL-800 Instruction Manual

Pin 8 of IC1 will go negative if the grid shunt voltage exceeds the voltage from divider R311 and

R2 (chassis mounted control), or if the supply voltage sags from excessive line voltage drop. Pin 8

is followed by current buffer Q303 that sources the negative ALC voltage to the ALC output jack.

Dynamic Bias

Conventional bias circuits allow high power linear amplifiers to dissipate hundreds of watts during

low or no signal periods. This creates needless heat, since virtually no dissipation is required unless

the amplifier is being driven with large signal levels.

The AL-800 contains an exclusive bias circuit that reduces the idling (quiescent) current very close

to the tube's cut off region. The power amplifier tube in the AL-800 has a full resting period of very

low dissipation between dots and dashes on CW and between words on SSB. The lower idling

current dramatically reduces component temperatures on both CW and SSB. If only a few

milliwatts of RF power are applied to the amplifier, the quiescent current will increase. Linearity

remains excellent with this circuit because the tube's conduction angle is never reduced below class

AB.

The dynamic bias circuit is located on the Power Supply Board P/N 50-0800-H1. Diodes D101 and

D102 rectify a small sample of the RF drive voltage. This voltage is applied to the base of dc switch

Q101. If Q101's base is driven with a few microamperes of current from the RF sampling circuit,

Q101's collector will pull the base of PNP transistor Q102 low. This turns dc switch Q102 on.

When Q102 is on, zener diode D103 is connected between the collector and base of Q103.

D103 sets the operating bias. This zener applies forward bias to Q103's base whenever the collector

voltage of Q103 exceeds the voltage of D103. This forward bias will turn Q103 on harder and will

reduce the collector voltage. If the collector voltage is less than the breakdown voltage of D103,

Q103 will move towards cut-off and the collector voltage will increase. Q103 is, in effect, a current

buffer for zener diode D103.

Timer - Overload Circuits

CAUTION:

Never modify or disable protection circuits without factory approval. Doing so

with void all manufacturer's warranties.

Grid Overload Protection

This amplifier contains a fast acting electronic grid protection circuit to enhance tube life. The

amplifier will quit operating and the

"OL"

and

"XMT"

LEDs will both illuminate if the safe limit

of grid dissipation is approached or exceeded. The grid overload protection circuit is reset whenever

the

"STBY-OPR"

switch is placed in the

"STBY"

position.

Note:

This circuit has a much faster response time than the grid meter. This circuit also

measures the peak grid current. The operator may not observe the grid current

meter climbing before the overload circuit responds and disables the amplifier.